In this lesson we will discuss the three laws of logic. Learn about each of the three laws, what they mean, and how they apply to rational thinking. Then test your knowledge with a quiz.

Origins of the Laws of Logic

There are three fundamental laws upon which logic and rational thinking are based. These three laws are thought to have originated with Aristotle, who believed that the laws are necessary conditions for rational thinking to occur. The three laws are the law of identity, law of non-contradiction, and law of the excluded middle. Let's examine each of the three laws of logic in more detail.

Law of Identity: X is X

The law of identity states that if a statement has been determined to be true, then the statement is true. For example, if I make a statement that 'It is snowing,' and it is the truth, then the statement must be true. If we look at the law of identity in more general terms, it says that each thing that exists is made up of its own particular characteristics that are a part of what it is.

When you apply this to logic, the law of identity essentially means that everything is itself and it cannot be something else. Snow cannot be clouds, and water cannot be a pole. Each thing is something specific that has a particular identity. So when I say that it is snowing, snowing refers to a particular event. Given that 'snowing' refers to a specific thing, if I make this statement while it is actually snowing, then it must be a true statement.

Law of Non-Contradiction: X is not non-X

According to the law of non-contradiction, a statement such as 'It is snowing' cannot be both true and false. This means that it cannot be both snowing and not snowing during the same time period in the exact same location. In other words, nothing that is true can contradict itself. The law of non-contradiction is very important. Without it, we would not be able to think rationally.

Law of the Excluded Middle: Either X or not X

The third and final law is the law of the excluded middle. According to this law, a statement such as 'It is snowing' has to be either true or false. Either it is snowing or it is not snowing; there are no other alternatives. According to this law, it is not possible for real contradictions to exist; the contradictions are the result of not having enough data or limitations in language. Therefore when presented with a green object and asked whether the object is blue or yellow, we should assume that the object is neither yellow nor blue but something else entirely: green.

Lesson Summary

There are three laws upon which all logic is based, and they are attributed to Aristotle. These laws are the law of identity, law of non-contradiction, and law of the excluded middle. According to the law of identity, if a statement is true, then it must be true. The law of non-contradiction states that it is not possible for a statement to be true and false at the same time in the exact same manner. The law of the excluded middle says that a statement has to be either true or false.

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